Knowledge of the pathological physiology of parasitic diseases and of differences between parasitic strains is advanced by studying the susceptibility of host tissues to parasitic invasion and assessing the alteration of various physiological and histochemical parameters. On the basis of significant increases in total plasma creatine phosphokinase (CPK), Y-glutamyl transpeptidase, and CPK isoenzymes, the present findings clearly demonstrate marked muscle damage in infections with the House 510 and House 11 strains of Trypanosoma cruzi and significant deterioration of liver in infections with the Tulahuen strain. The occurrence of at least one additional isoenzyme, more cathodic than LDH-5, identified as LDH-X, was associated with testicular and epididymal tissues. Corresponding histopathological changes indicating injury in these organs were observed. These findings indicate the usefulness of isoenzyme studies in assessing tissue injury in parasitic infections and in the differentiation of parasitic strains, e.g., myotropic vs reticulotropic strains of T. cruzi.